<Query Kind="Expression" />

public static class DateTimeExtensions
{
	public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
	{
		int diff = dt.DayOfWeek - startOfWeek;
		if (diff < 0)
		{
			diff += 7;
		}
		return dt.AddDays(-1 * diff).Date;
	}
}